Water borne polymers prepared by emulsion polymerization are extremely important commercial materials. Because of the dual phase nature of these materials they require a stabilizing package to maintain water dispersibility. Often stability is maintained through the addition of surfactants or water soluble polymers such as polyvinyl alcohol and cellulose. In addition to maintaining water dispersibility, these stabilizers offer other end use advantages in specific applications.
Emulsion polymers have been prepared in the presence of water dispersible polyurethanes. Typically, these polymers are referred to as hybrids combining features of each while differing from the features of blends. For the most part, such water dispersible polyurethanes have been anionic polyurethanes incorporating amine neutralized carboxyl functionality. More recently, some nonionic polyurethanes have been utilized to stabilize the emulsion polymerization of a variety of monomers. Polyurethane dispersions also have been post added to emulsions to improve film coalescence and add toughness, but this approach is often hindered by lack of emulsion stability when the two components are mixed together and also by incompatibility of the urethane and vinyl polymer resulting in cloudy films and poor mechanical properties.
Representative patent literature which show anionic water dispersible polyurethanes and hybrid systems are as follows:
U.S. Pat. No. 5,173,526 discloses a process for producing an aqueous polyurethane-vinyl polymer (hybrid) coating composition by (a) forming a carboxy-containing, water-dispersible, isocyanate-terminated polyurethane prepolymer, (b) adding a vinyl monomer mixture to the carboxy-containing, water-dispersible, isocyanate-terminated polyurethane prepolymer to make a prepolymer/monomer mixture, (c) adding tertiary amine to the prepolymer/monomer mixture, (d) dispersing the prepolymer/monomer mixture in water, (e) adding an oil-soluble free radical initiator and a chain extender to the aqueous dispersion and (f) completing the chain extension and polymerizing the vinyl monomers by heating the aqueous dispersion. A variety of vinyl monomers, which include lower alkyl (C.sub.1-6) esters of acrylic or methacrylic acid, are suggested as being suited for the hybrid.
U.S. Pat. No. 4,927,876 discloses two types of aqueous polymer dispersions for use as coating compositions, the dispersions comprising a water dispersible polyurethane containing diphenylmethane diisocyanate and a vinyl polymer. The water dispersible polyurethane component is prepared by forming an isocyanate terminated prepolymer having carboxyl functionality incorporated therein and chain extending with an amine. In forming the aqueous dispersion, the vinyl polymer may be blended with the water dispersible polyurethane or the vinyl monomer may be polymerized in the presence of the aqueous polyurethane dispersion. A variety of monomers suited for polymerization are suggested which include acrylates and hydroxyacrylates.
Nonionic, water dispersible polyurethanes have been utilized in the formulation of coatings for wood, metals and flexible substrates. Representative examples are set forth in the following patents:
U.S. Pat. No. 4,190,566 discloses nonionic, water dispersible polyurethanes having a linear molecular structure. The nonionic, water dispersible polyurethanes are formed by reacting organic diisocyanates with difunctional organic compounds having molecular weights from about 300 to 6,000 in the presence of components having hydrophilic groups, e.g., conventional polyether and polyester polyols, guaranteeing the dispersibility of the polyurethanes. The hydrophilic components are based on reacting alkylene oxides with monofunctional alcohols or any alternative reacting monoisocyanate with the polyether alcohol. Optionally, the water dispersible polyurethanes may be blended with other vinyl polymers for the formulation of coatings.
U.S. Pat. No. 4,992,507 discloses aqueous dispersions of nonionic, water dispersible polyurethanes having pendent polyoxyethylene chains and free acid or free tertiary amino groups. These water dispersible polyurethanes are formed by reacting inorganic polyisocyanate with an organic polyol and a dispersing diol or diisocyanate having pendant polyoxyethylene chains and an isocyanate reactive compound containing at least a carboxylic acid group. Examples of isocyanate reactive compounds having carboxy groups include 2,2-dimethylolpropionic acid. In this polymer, the acid remains as the free carboxy group rather than the salt. Typically, the carboxy functionality is reacted with amine to form the anionic water dispersible polyurethanes.
WO 94/13726 discloses a class of hydrophilic polyurethanes which are water dispersible. These polyurethanes are high viscosity, water dispersible polyurethanes, and these are formed by reacting aliphatic or aromatic diisocyanates with polyethylene glycol which are polyaddition products of ethylene oxide and a variety of divalent alcohols, a representative is the reaction product of a polyethylene glycol with meta-tetramethylxylene diisocyanate. The patent also discloses the use of these high viscosity polyurethanes as protective colloids in combination with conventional nonionic surfactants for use in stabilizing the emulsion polymerization of monomers such as vinyl acetate and vinyl chloride. The patentees point out that the resulting polyvinyl acetate results in the production of water-resistant films.
DE 195 08 856 discloses the use of water soluble-nonionic polyurethanes as protective colloids of the type described in WO 94/13726 for the emulsion polymerization of a variety of monomers. Representative monomers suited for forming aqueous dispersions include acrylic and methacrylic acid esters, acrylic and methacrylic acids as well as their hydroxy alkyl esters, vinyl acetate and copolymers of vinyl acetate and ethylene. The patentees also point out that redispersible powders can be obtained from these emulsions. Representative examples then show the use of the water soluble polyurethanes as protective colloids alone and in combination with surfactants for the polymerization of ethylene, vinyl acetate and N-methylol acrylamide.
U.S. Pat. No. 5,314,942 discloses the preparation of aqueous polymer dispersion containing vinyl polymer and a nonionic water-dispersible polyurethane having pendent polyoxyethylene chains. The nonionic polyurethanes having pendent polyoxyethylene chains are alleged to possess significant advantages over the anionic polyurethanes described in the art. Acrylic, substituted acrylic and vinyl esters are alleged for use in emulsion polymerization. The ratio of polyurethane to vinyl polymer is within a weight range of 1:9 to 9:1. Most of the examples show the polymerization of acrylic monomers in the presence of the nonionic dispersion and sodium lauryl sulfate.